Nothing wrong with socket 775 cpu's and the 8 series cpu is decent piece of silicon, like most 775's it overclocks well.
That said socket 775 is a dead end street now, the cpu's you see in the line up are the last of their kind. if you go for i3 or i5 (depending on your budget) you will be buying into the next generation of cpus which have only just started to show their worth, hence you will be able to upgrade a lot over the comming years something you will not be able to do with 775.
AM3 is only really an option for the budget concious buyer. the cpu line up is ok but until the next generation show their colours AMD are looking a little bit tired. the AM3 lineup compares reasonably well with 775 but doesnt match the capabilities of the i3 and i5 lineup. be interesting to see AMD's new on die gpu chipos when they arrive, heres hoping they work on AM3 boards and AMD dont change the socket (again).
Graphics wise the intel 4500 is a reasonable onboard solution BUT if you dont get the HD version you will not get HD playback. you will want this when you want to play your blue ray movies... that said a cheap ATi or nVidia PCie graphics card wont cost the earth if you dont want to play games.
the intel 4500 will handle the Win 7 aero interface with no problems and will be ideal for general useage.
Make sure you have a decent branded psu (cant remember what you said you had) dont touch winpower with a bargepole. decent psus will save your pc from imminent irreversable destruction and will keep your pc quiet.
Buy yourself a decent aerocool cpu cooler to replace the intel trite, this will keep the cpu a lot cooler if you overclock and will keep the case quieter.
good luck with it.